In devices for writing onto media using a scanning light beam it is important to know when to turn on and start writing with the scanning light beam. In a typical prior art scanning writing apparatus, a laser light beam or other illuminated light, is directed to a rotating polygon having a plurality of mirror facets which direct the light beam onto a media. Each facet is used to write a successive line of data. These type of systems have the disadvantage of relying on ill-defined specifications for rotating stability of the polygon. Additionally, accumulative error in timing from the first facet to the last facet may occur. Thus, there is a need to provide appropriate timing for line start for each successive facet.
It is also important to maintain a predetermined intensity level of the writing beam. As is typical with writing beams, such as laser beams, over time the intensity can vary, which can affect the overall quality of the writing apparatus. Most systems monitor light beams in the static mode, that is, before scanning. Monitoring may be done right at the output of the laser, or after the acoustic optical modulator (AOM) or attenuator. A hand-held light meter can periodically be inserted into the beam for measuring of the intensity. The problem is that measuring the intensity at these positions does not actually reflect the amount of light that will be hitting the media plane, which may result in erroneous measurements. The intensity of the beam should be measured as close to the writing plane as possible in order to account for all possible attenuation. Another prior art method used to monitor the intensity is to scan and print an image onto a photosensitive material media, for example, photographic paper, then develop the paper and then measure the results of the developed paper with a densitometer. This is a tedious and time-consuming process.
The present invention provides a method for accurately determining a line start for each facet and also provides a method of measuring the light beam amplitude automatically and as close to the focal plane of the media as possible.